better usability of Casper.viewport()

@@ -675,11 +675,18 @@
         /**
          * Changes the current viewport size.
          *
-         * @param  Object  viewportSize  An objet with two props: width and height
+         * @param  Number  width   The viewport width, in pixels
+         * @param  Number  height  The viewport height, in pixels
          * @return Casper
          */
-        viewport: function(viewportSize) {
-            this.page.viewportSize = viewportSize;
+        viewport: function(width, height) {
+            if (typeof width !== "number" || typeof height !== "number" || width <= 0 || height <= 0) {
+                throw new Error("Invalid viewport width/height set: " + width + 'x' + height);
+            }
+            this.page.viewportSize = {
+                width: width,
+                height: height
+            };
             return this;
         },
 

@@ -51,10 +51,10 @@ casper.test.assert(casper.steps.length === 1, 'start() can add a new navigation
 
 // Casper.viewport()
 casper.test.comment('viewport');
-casper.viewport({width: 1337, height: 999});
-casper.test.assertEquals(casper.page.viewportSize.width, 1337, 'Casper.viewport() can change the width of the viewport');
-casper.test.assertEquals(casper.page.viewportSize.height, 999, 'Casper.viewport() can change the height of the viewport');
-casper.test.assertRaises(casper.viewport, 'booh', 'Casper.viewport() validates viewport size data');
+casper.viewport(1337, 999);
+casper.test.assertEquals(casper.page.viewportSize.width, 1337, 'Casper.viewport() can change the width of page viewport');
+casper.test.assertEquals(casper.page.viewportSize.height, 999, 'Casper.viewport() can change the height of page viewport');
+casper.test.assertRaises(casper.viewport, ['a', 'b'], 'Casper.viewport() validates viewport size data');
 
 // Casper#then()
 casper.test.comment('then');